Method for placing cement around casing

ABSTRACT

Casing to be set in a well is lowered to a position just above the interval of the well in which cementing is critical. Small diameter tubing having scratchers mounted on its outer surface is lowered through the casing and reciprocated vigorously while circulating liquid to remove soft filter cake and cuttings from the borehole wall through the critical area. A cement slurry is pumped down the tubing to puddle cement slurry in the bottom of the borehole. The casing is then lowered into the puddled cement slurry. A float valve on the lower end of the casing causes the cement slurry to be displaced up into the annulus surrounding the casing.

Unite will [72] Inventor Robert J. Goodwin Oakmont, Pa. [2]] Appl. No.880,060 [22] Filed Nov. 26, 1969 [45] Patented Apr. 27, 1971 [73]Assignee Gulf Research & Development Company Pittsb g Pa.

[54] METHOD FOR PLACING CEMENT AROUND CASING 8 Claims, 3 Drawing Figs.

[52] 11.8. C1 166/290 [51] lnt.Cl E2lb 33/14 [50] Field of 166/285, 290,173

[56] References Cited UNITED STATES PATENTS 2,107,327 2/1938 Creighton166/290 2,416,842 3/1947 OLeary 166/290X 2,421,434 6/1947 Reistle et al.166/173 3,417,816 12/1968 Morris et al. 166/290 Primary Examiner-Ian A.Calvert Attorneys-Meyer Neishloss, Deane E. Keith and Paul L.

Tillson ABSTRACT: Casing to be set in a well is lowered to a positionjust above the interval of the well in which cementing is critical.Small diameter tubing having scratchers mounted on METHOD FOR PLACWGCEMENT AROUND CASING This invention relates to wells for the productionof fluids from subsurface formations and more particularly to a methodof cementing casing in wells.

It is the usual practice in drilling wells for the production of oil andgas to set several concentric strings of casing in the well. Each stringof easing extends deeper in the well than the next larger string ofcasing. The casing program is designed to isolate certain formationspenetrated by the well from other formations. After a string of casingis lowered into the well a cement slurry is pumped down the casing andforced upwardly through the annulus between the casing and the boreholewall. The casing is suspended in the well until the cement sets to forma solid sheath filling the annulus surrounding the casing. The cementsheath is designed to seal the annular space between the casing and theborehole wall and thereby prevent flow of gas or oil into water-bearingformations penetrated by the well or water into gasand oil-bearingformations. Slurries of Portland cement are usually used to form thedesired cement sheath in the annulus surrounding the casing.

Often the cement sheath does not completely fill the annular spacebetween the outer surface of the casing and the borehole wall, and thecement then does not isolate successive formations in the mannerdesired. One reason for unsatisfactory cement jobs is the tendency ofthe cement slurry to channel through the space surrounding the casingwhen the cement slurry is pumped down the casing and displaced upwardlythrough the annulus surrounding the casing. Unsatisfactory cementingjobs are also caused by the drilling mud filter cake on the boreholewall at the end of the drilling operation preventing adherence of cementto the borehole wall.

Some operators have specified that scratchers be attached to the outersurface of the casing and the casing vigorously moved either byreciprocating or rotating while cement is pumped into place. Thescratchers are designed to scrape soft mud cake and loose cuttings fromthe borehole wall and in conjunction with the vigorous movement of thecasing is believed to prevent channeling through the annulus between thecasing and the borehole wall. It has been found in the field that as apractical matter a thorough scratcher job by rotation or reciprocationof casing is rarely accomplished. Rig crews are reluctant to reciprocatelarge casing rapidly enough to prevent channeling and cause the removalof the cuttings and drilling mud filter cake necessary to obtain astrong bond of the cement to the borehole wall. Moreover, the verystrong plunger effect resulting from reciprocation of casing mayfracture the well or reduce the hydrostatic pressure in the well to suchan extent that gas enters the well and causes a blowout.

To eliminate the disadvantages of conventional cementing methods it hasbeen suggested that the cement slurry be puddled in the bottom of theborehole and casing then run into the puddled cement. If the cement isdelivered into the bottom of the borehole through small diameter tubinghaving scratchers attached to its outer surface and the tubingreciprocated vigorously while circulating liquids in the hole, bothdrilling mud and mud filter cake can be effectively removed from thecritical area and a uniform cement sheath strongly adhering to theborehole wall can be obtained. Rig crews do not object to rapidreciprocation of small diameter tubing such as drill pipe. However, thetime required to run casing into the well after placement of the cementmakes it necessary to use highly retarded cements to avoid setting ofthe cement before the casing is in place, and such retardation may causeexcessive time waiting on cement.

This invention resides in a method of cementing casing in a well inwhich casing is run into the borehole to a depth such that the lower endof the casing is just above the level to which a strong uniform cementsheath is required. A small diameter pipe having scratchers on its outersurface is run through the casing and the float valve and reciprocatedor rotated vigorously while a liquid is pumped through it to removedrilling mud and mud filter cake from the hole. When returns at thesurface indicate that the borehole is clean the desired amount of cementis pumped down the small diameter pipe and discharged therefrom into thebottom of the hole while reciprocation and/or movement of the smalldiameter pipe is continued. The small diameter pipe is then withdrawn,and the casing lowered into the cement slurry and suspended in thecement until the cement sets.

Referring to the drawings:

FIG. 1 is a diagrammatic view partially in vertical section of the lowerend of a borehole treated in accordance with this invention during theperiod of cleaning the borehole for the cementing operation.

FIG. 2 is a diagrammatic view partially in vertical section showing thewell after placement of the cement in its lower end.

FIG. 3 is a diagrammatic view partially in vertical section showing thewell at the end of the cementing operation.

Referring to FIG. 1 of the drawings, the lower end of the borehole 10 ofa well is illustrated penetrating a productive fluid-bearing formationl2 and extending to a total depth 14 in a formation 16 underlyingformation 12. A formation 18 overlies formation 12. The lower end of apreviously set string of casing 19 is shown set in the overlyingformation 18. It is desired to set a string of casing 20 in the wellwith the cement sheath surrounding the casing 20 through formations l6and 12 and upwardly through formation l8'for an interval insuringisolation of formation 12 from overlying formations after the cementingoperation is completed.

Casing 20 is run into the well to a depth placing the lower end 22 ofthe casing to be cemented just above the critical interval through whicha continuous cement sheath is desired. Mounted on the lower end ofcasing 20 is a float valve 24 illustrated as being of the flapper typeadapted to close the lower end of the casing 20 as it is run into thewell and during subsequent steps in this cementing process. Casing 20has centralizers 25 along its length to keep the casing away from theborehole wall.

A small diameter tubing 26, which may be drill pipe, is run into thewell through casing 20 and downwardly through float valve 24. Tubing 26should have a diameter less than about 50 percent of the diameter ofborehole 10. Small diameter tubing 26 has a plurality of scratchers 28mounted at intervals along its outer surface illustrated in the drawingsas the type operated by reciprocation of the tubing. Other types ofscratchers can be used. After small diameter tubing 26 has been run to adepth placing the uppermost scratchers 28 below the float valve 24 adistance such that the scratchers will not engage the float valve duringthe scratching operation and the lower end below the formation 12, thesmall diameter tubing 26 is reciprocated while a liquid is pumpeddownwardly through the tubing 26 into the bottom of the borehole anddisplaced upwardly through the annulus 30 between the outer surface ofcasing 20 and the borehole wall 32. It is preferred that suitablesealing means, not shown, be provided in casing 20, preferably at itsupper end, to engage the outer surface of tubing 26 and thereby forcethe displaced liquid to move up through the annulus 30. Reciprocation ofthe tubing 26 should be through a stroke long enough to insurescratching of the borehole wall through the entire interval throughwhich a strong bond is essential. Circulation and reciprocation can becontinued for any desired length of time. it is preferred to continuethe circulation until returns at the surface indicate that soft filtercake and loose cuttings on the borehole wall have been removed over thearea scratched. Any liquid can be circulated to accomplish the desiredborehole wall cleaning. While a drilling mud can be used and is quiteoften necessary, it is preferred to use a clear liquid to avoid buildingadditional filter cake during the scratching operation.

After the borehole wall is clean a cement slurry is pumped down throughsmall diameter tubing 26 and discharged into the lower end of theborehole while, preferably, continuing reciprocation of the tubing. Thevolume of cement pumped into the hole will depend upon the length of theinterval to be cemented. In FIG. 2 the volume of cement is illustratedas filling the borehole to the level 34. After the desired volume ofcement has been displaced into the hole, the small diameter tubing iswithdrawn through the casing, and the casing is lowered to the desireddepth in the well below the formation 12. The float valve on the lowerend of the casing prevents entry of cement into the casing whereuponlowering of the casing displaces cement upwardly through the annulussurrounding the casing. Since the float valve is not necessary until thecasing is lowered into the puddled cement slurry, float valve 24 may beof a type that can be lowered into position on a wire line after smalldiameter tubing 26 has been pulled from the well. Casing 20 is thensuspended in the well until the cement in the annulus sets.

Cement slurry fills the annulus above the level 34 in the same manner asin conventional cementing operations, but the annulus through thecritical interval is freed of drilling mud and drilling mud solids bythis invention. This cementing method insures a good bond throughout theinterval in which a good cementing job is required by removing the mudfilter cake and any liquid mud from the borehole throughout the intervalin which a good bond is required. The positioning, before the placementof cement, of the lower end of the casing immediately above the intervalin which a good bond is required allows the casing to be quickly loweredinto the desired final position after the cement has been delivered intothe hole, and thereby makes unnecessary extensive retarding of thecement. It is then not necessary to conduct further steps designed totrigger the setting of a highly retarded cement to avoid excessive lossof rig time while waiting for the cement to $16!.

The location of the lower end of the casing during scratching andplacement of the cement will depend on the conditions in the particularwell. If, for example, formation 12 is a very high-pressure formation,it may be desirable to insure removal of drilling mud and mud filtercake from the borehole for an interval of L000 feet above the upperboundary of the critical formation, but in other wells shorter intervalsas small as 50 feet may be adequate. In any event, the time required torun the casing to the desired final depth after placement of the cementcan be greatly reduced by running the casing to just above the criticalinterval before placement of the cement. It is then not necessary toretard the setting of the cement greatly, and the danger ofoverretarding the cement is eliminated.

in some wells, particularly deep wells, there will be intermediatestrings of casing that extend substantial distances down the well. Thecasing to be set by the process of this invention can advantageously besuspended in the well with its lower end at or just slightly above thelower end of the next larger string of easing during the scratching,circulating, and cement puddling operations. in that manner all chancesof the casing becoming stuck during those operations can be avoided.While the reduction in the time required to lower the casing to itsfinal position may be lessened, important reductions in time are stillpossible if the intermediate string of casing goes to a substantialdepth, and this embodiment is particularly advantageous when theintermediate casing extends for a depth of 3,000 feet or more.

lclaim:

l. A method of cementing casing in the borehole of a well comprisingrunning casing into the borehole and suspending the casing with itslower end a short distance above the interval of the borehole in whichcementing is critical and not substantially higher than the lower end ofthe next larger string of casing, running small diameter tubing havingscratchers on its outer surface through the casing into the boreholebelow the casing, circulating a liquid down through the tubing into theborehole of the well near the bottom of the well and up the well whilemoving the tubing to remove drilling mud and drilling mud filter cakefrom the critical interval of the borehole, discharging from the smalldiameter tubing into the lower end of the borehole a cement slurry in anamount which will be sufficrent to fill the annular space between thecasing and the borehole wall through the desired interval, withdrawingthe small diameter tubing through the casing and closing the lower endof the casing, lowering the casing into the cement slurry to the desireddepth in the borehole, and maintaining the casing at the desired depthuntil the cement sets.

2. A method as set forth in claim 1 in which the casing is initiallysuspended with its lower end at least 50 feet above a fluid-bearingformation to be isolated by the casing.

3. A method as set forth in claim 1 in which the cement slurry is aslurry of Portland cement.

4 A method as set forth in claim 1 in which the small diameter tubing isreciprocated in the hole to scratch the borehole wall through the entirecritical interval.

5. A method as set forth in claim 1 in which the small diameter tubingis rotated, and the scratchers are adapted to scratch the borehole wallthrough the critical area on rotation of the small diameter tubing.

6. A method as set forth in claim 1 in which the small diameter tubingis drill pipe.

7. A method as set forth in claim 1 in which centralizers are mounted onthe casing at intervals over that portion of the casing opposite theborehole wall.

8. A method of cementing casing in a well the borehole of which extendsa substantial distance below intermediate casing extending at least3,000 feet into the well comprising running the casing into theintermediate casing to locate the lower end of the casing atsubstantially the depth of the lower end of the intermediate casing,running small diameter tubing having scratchers mounted on its outersurface through the casing and into the borehole below the intermediatecasing, circulating liquid through the small diameter tubing whilemoving the small diameter tubing through the interval of the borehole inwhich cementing of the casing is critical to remove cuttings and mudfilter cake from that interval of the borehole, discharging from thesmall diameter tubing into the lower end of the borehole a cement slurryin an amount which will be sufficient to fill the annular space betweenthe casing and the borehole wall through the desired interval, pullingthe small diameter tubing from the well and closing the lower end of thecasing, lowering the casing into the cement slurry to the desiredposition in the well, and suspending the casing at the desired positionuntil the cement sets.

1. A method of cementing casing in the borehole of a well comprisingrunning casing into the borehole and suspending the casing with itslower end a short distance above the interval of the borehole in whichcementing is critical and not substantially higher than the lower end ofthe next larger string of casing, running small diameter tubing havingscratchers on its outer surface through the casing into the boreholebelow the casing, circulating a liquid down through the tubing into theborehole of the well near the bottom of the well and up the well whilemoving the tubing to remove drilling mud and drilling mud filter cakefrom the critical interval of the borehole, discharging from the smalldiameter tubing into the lower end of the borehole a cement slurry in anamount which will be sufficient to fill the annular space between thecasing and the borehole wall through the desired interval, withdrawingthe small diameter tubing through the casing and closing the lower endof the casing, lowering the casing into the cement slurry to the desireddepth in the borehole, and maintaining the casing at the desired depthuntil the cement sets.
 2. A method as set forth in claim 1 in which thecasing is initially suspended with its lower end at least 50 feet abovea fluid-bearing formation to be isolated by the casing.
 3. A method asset forth in claim 1 in which the cement slurry is a slurry of Portlandcement.
 4. A method as set forth in claim 1 in which the small diametertubing is reciprocated in the hole to scratch the borehole wall throughthe entire critical interval.
 5. A method as set forth in claim 1 inwhich the small diameter tubing is rotated, and the scratchers areadapted to scratch the borehole wall through the critical area onrotation of the small diameter tubing.
 6. A method as set forth in claim1 in which the small diameter tubing is drill pipe.
 7. A method as setforth in claim 1 in which centralizers are mounted on the casing atintervals over that portion of the casing opposite the borehole wall. 8.A method of cementing casing in a well the borehole of which extends asubstantial distance below intermediate casing extending at least 3,000feet into the well comprising running the casing into the intermediatecasing to locate the lower end of the casing at substantially the depthof the lower end of the intermediate casing, running small diametertubing having scratchers mounted on its outer surface through the casingand into the borehole below the intermediate casing, circulating liquidthrough the small diameter tubing while moving the small diameter tubingthrough the interval of the borehole in which cementing of the casing iscritical to remove cuttings and mud filter cake from that interval ofthe borehole, discharging from the small diameter tubing into the lowerend of the Borehole a cement slurry in an amount which will besufficient to fill the annular space between the casing and the boreholewall through the desired interval, pulling the small diameter tubingfrom the well and closing the lower end of the casing, lowering thecasing into the cement slurry to the desired position in the well, andsuspending the casing at the desired position until the cement sets.